There are position-coding patterns which have the property that the absolute position in the position-coding pattern for a partial area of the position-coding pattern of a smallest predetermined size can be calculated by means of the information in the partial area and a predetermined algorithm. Examples of such position-coding patterns are disclosed in U.S. Pat. No. 5,477,012 (O. Sekendur), where each position is coded by means of a symbol, and WO 92/17859 (Hewlett Packard Co.), WO 00/73983 (Anoto AB), WO 01/26033 (Anoto AB) and U.S. Pat. No. 5,973,110 (Xerox Corp), where each position is coded by means of a plurality of symbols.
Position-coding patterns of this type can be used, for instance, for digital recording of information which is written and/or drawn by hand by means of a user unit on a base provided with the position-coding pattern. While moving the user unit, images of the position-coding pattern are continuously recorded locally at the tip of the user unit. One or more symbols in each of the images are decoded to a position. The decoded positions together constitute a digital description of the user unit's movement over the base.
As long as the position-coding pattern is perfectly reproduced on the base and as long as the position-coding pattern is perfectly imaged in the recorded images, the position decoding can be expected to function very well. In practice, however, there is a risk of the position-coding pattern being distorted more or less when being applied to the base. It also happens that the user unit does not image the position-coding pattern perfectly owing to defects in the user unit's imaging system and because the user tilts the user unit when writing and the position-coding pattern thus is imaged in perspective. This can in turn result in incorrect positions being decoded from the position-coding pattern.
Incorrectly decoded positions can be discovered if the position-coding pattern contains redundant information which enables error detection. They can also be discovered if the distance to previously and/or subsequently decoded positions is unreasonable.
If an incorrectly decoded position is detected, it can be either retained or rejected. If the position is retained, the digitally recorded positions will not correctly represent the user unit's movement over the base. If the incorrectly decoded position is instead rejected, there will be a “gap” in the digitally recorded positions. Many such gaps can obstruct the possibility of interpreting what has been written on the base.